An economical route for converting carbon dioxide to value-added organic compounds would be highly desirable because of the role carbon dioxide (CO2) plays in global climate change, and in the depletion of fossil fuel resources. Although CO2 is an inexpensive, non-toxic, abundant carbon feedstock, it is difficult to economically reduce CO2 to a more useful form because of its thermodynamic stability and kinetic inertness. For CO2 reduction to be attractive on a large scale, the process needs to work under mild reaction conditions, and the process must be economical.
CO2 has traditionally been captured by absorption into a solution of an organic amine. This method is energy-intensive; it requires heating the solution to disperse the absorbed CO2 for storage. If the absorbed CO2 is simply driven off, then it must be stored somewhere (e.g. in an underground rock formation) to avoid release into the atmosphere.
Other methods that have been tried include activating and reducing CO2 by electrochemical and electrocatalytic means in the presence of various transition metals and alloys.
There have been reports of using low-valent d-block and f-block metal complexes to reduce CO2 to oxalate. Horn, B., Limberg, C., Herwig, C. & Braun, B. Nickel(I)-mediated transformations of carbon dioxide in closed synthetic cycles: reductive cleavage and coupling of CO2 generating NiICO, NiIICO3 and NiIIC2O4NiII) entities. Chem. Commun. 49, 10923-10925, doi:10.1039/C3cc45407j (2013) reported the use of β-diketiminate-based nickel(I) complexes to reduce CO2 to CO or C2O42− in two closed synthetic cycles. A significant limitation of the Horn et al. system was its use of KC8, one of the strongest reducing agents available, to reduce Ni(II) to Ni(I).
Angamuthu, R., Byers, P., Lutz, M., Spek, A. L. & Bouwman, E. Electrocatalytic CO2 Conversion to Oxalate by a Copper Complex. Science 327, 313-315, doi:10.1126/science.1177981 (2010) reported a binuclear copper(I) complex that can reduce CO2 to oxalate, forming a tetranuclear copper(II) oxalate complex. Oxalate was then released by electrolysis, using lithium perchlorate as the supporting electrolyte, to complete the electrocatalytic cycle. The complex contained an amino-acid-derived ligand that bound two Cu atoms. Two of the complexes reacted with four CO2 molecules to form two oxalates: 2Cu2L+4CO2→Cu4L2(C2O4)2.
Crowley, J. D.; Bandeen, P. H., A multicomponent CuAAC “click” approach to a library of hybrid polydentate 2-pyridyl-1,2,3-triazole ligands: new building blocks for the generation of metallosupramolecular architectures. Dalton Trans. 2010, 39, 612-623; doi: 10.1039/B911276F discloses a CuAAC reaction for generating alkyl, benzyl or aryl linked polydentate pyridyl-1,2,3-triazole ligands from the corresponding halides, sodium azide, and alkynes. Complexes with Ag(I) were described.
Other methods to reduce CO2 include electrochemical or photochemical processes. In the Bocarsly “liquid light” approach, CO2 reacts with an electrochemically-reduced solution of a heterocyclic amine such as pyridine. In photochemical reduction, some or all of the energy needed for CO2 reduction is supplied by light.
Reductive dimerization of carbon dioxide to oxalate (C2O42−) converts an environmental pollutant into a more useful organic compound. There is an unfilled need for improved, economical methods to reduce CO2 to oxalate. If a suitable chemistry could operate rapidly and cleanly enough, then it could be used to capture CO2 from the atmosphere or from other chemical processes (e.g. combustion, cement manufacture). Oxalate and oxalic acid have many uses, including in extractive metallurgy, as mordants in dyeing processes, as bleaching agents, as miticides, and as reagents in various synthetic processes. In very large quantities, oxalate may also simply be used to sequester CO2 in solid form, e.g. as CaC2O4.